It looks like it should work alright, though I would recommend not routing both connectors to the card (i.e., connect the 6-pin from the power supply like normal, then do the 8-pin from the adapter, so that you essentially end up with the two 6-pin lines remaining separate). It probably wouldn't explode outright or anything, but unless you're anal about cable management it would probably be best to have them separate (though you could always try it and see). Near as I can find, the +12v rail is split three ways with 18 amps on each (rather than it all being one rail).